The field of the invention is industrial control systems such as process controls, numerical controls, and programmable controllers, and particularly, to local area networks for connecting such controls together.
Local area networks are comprised of two or more nodes, or stations, which are linked together by a communications media. The communications media may take a number of forms, including coaxial cable, fiber optics or twisted wire pairs. The topology of the links between stations may also take a number of forms, including star, multidrop or ring configurations.
Regardless of the media used or the topology of the network, a control scheme is required to provide an orderly transfer of information from one station to another on the network. The most primitive schemes employ a master station which is responsible for controlling the operation of the network. The master may "poll" each slave station on the network for information to be delivered to other stations, or it may enable a polled station to transmit its message directly to other stations. Such networks are not appropriate for industrial applications because the entire network is brought down if a malfunction should occur in the master station.
Another solution is to employ a carrier sense multiple access (CSMA) scheme such as that defined by the Ethernet standards. In CSMA networks each station waits for the network to go silent, and if it has a message to send, it takes control of the network and begins transmission. A mechanism must be provided to detect "collisions" when two or more stations transmit messages at the same time, and for contending with those stations to determine which has priority. While CSMA networks are appropriate for office and commercial applications, they are not appropriate for industrial networks which convey information required by "real time" control systems. Other, more "deterministic" network control schemes are required for industrial networks.
Other, more deterministic network control schemes include time slot reservation, slotted rings, register insertion rings and token passing networks. With a token passing scheme, a token in the form of a distinctive bit pattern is passed between stations on the network. While a station has the token, it can transmit messages to other stations or command other stations to transmit messages. While token passing schemes are simple in concept, they are complicated to implement due to the problem of lost tokens and duplicate tokens. These problems are more severe in the industrial environment where power may be lost, where stations may be added or removed from the network, or where malfunctions may occur in a station or the media connecting the stations.